Heretofore, as a method of forming a photoresist on the surface of a copper substrate, a method of coating a liquid photoresist, a method of laminating a dry film photoresist, and an electrodeposition coating method have been known. These methods, however, have respective problems.
The liquid photoresist coating method usually involves a photoresist composition that is generally dissolved in an organic solvent, coated on the surface of a copper substrate by a technique such as a spin coating method, a dip coating method, a screen printing method, a roller coating method, or a curtain coating method, and, thereafter, the organic solvent is evaporated to obtain a photoresist. In accordance with this method, although the production cost is low, when it is applied to a substrate for print wiring which has irregularities in the surface thereof, it is difficult to coat in a uniform thickness, which causes problems such as unevenness in development. Moreover, since an organic solvent is used, it is necessary to take precautions with respect to the dangers of explosion and other hazards. Furthermore, a sophisticated coating technique is demanded in order to obtain uniformity of the surface state, thickness, developing characteristics, sensitivity, and the like. Therefore, it is difficult to obtain high reproductivity.
For the foregoing reasons, the dry film photoresist method is mainly employed. Details of the liquid photoresist method and the dry film photoresist method are described, for example, in W. S. De Forest, Photoresist, 1975, McGraw Hill, New York.
In accordance with .the dry film photoresist method, a relatively thick photoresist layer is laminated on the surface of a copper substrate, and thus a uniform photoresist layer can be formed relatively easily. Thus the dry film photoresist method has an advantage in that a pattern can be obtained with high precision. However, the dry film photoresist method also has some disadvantages in that productivity is low and production cost is relatively high.
The electrodeposition coating method is a method to deposit an aqueous dispersion of a photoresist composition on a copper substrate as an anode while passing electricity therethrough. This method is described in JP-A-62-262855 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-62-262856 (corresponding to U.S. Pat. No. 4,845,012). Although this method has advantages of being high in resolving power and being low in production cost, because a photoresist in a thin film form can be obtained, it has a problem in respect of stability of the dispersion, and needs an apparatus for passing electricity.